Corrosion resistant PEM fuel cell

ABSTRACT

A PEM fuel cell having electrical contact elements comprising a corrosion-susceptible substrate metal coated with an electrically conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles. The substrate may have an oxidizable metal first layer (e.g., stainless steel) underlying the polymer coating.

The Government of the United States of America has rights in thisinvention pursuant to contract No. DE-AC02-90CH10435 awarded by theUnited States Department of Energy.

TECHNICAL FIELD

This invention relates to PEM fuel cells, and more particularly tocorrosion-resistant electrical contact elements therefor.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehiclesand other applications. One known fuel cell is the PEM (i.e., ProtonExchange Membrane) fuel cell that includes a so-called“membrane-electrode-assembly” comprising a thin, solid polymermembrane-electrolyte having an anode on one face of themembrane-electrolyte and a cathode on the opposite face of themembrane-electrolyte. The anode and cathode typically comprise finelydivided carbon particles, very finely divided catalytic particlessupported on the internal and external surfaces of the carbon particles,and proton conductive material intermingled with the catalytic andcarbon particles. One such membrane-electrode-assembly and fuel cell isdescribed in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assignedto the assignee of the present invention. Themembrane-electrode-assembly is sandwiched between a pair of electricallyconductive contact elements which serve as current collectors for theanode and cathode, and may contain appropriate channels and openingstherein for distributing the fuel cell's gaseous reactants (i.e., H₂ &O₂/air) over the surfaces of the respective anode and cathode.

Bipolar PEM fuel cells comprise a plurality of themembrane-electrode-assemblies stacked together in electrical serieswhile being separated one from the next by an impermeable, electricallyconductive contact element known as a bipolar plate or septum. Theseptum or bipolar plate has two working faces, one confronting the anodeof one cell and the other confronting the cathode on the next adjacentcell in the stack, and electrically conducts current between theadjacent cells. Contact elements at the ends of the stack contact onlythe end cells and are referred to as end plates.

In an H₂-O₂/air PEM fuel cell environment, the bipolar plates and othercontact elements (e.g., end plates) are in constant contact with highlyacidic solutions (pH 3-5) containing F⁻, SO₄ ⁻⁻, SO₄ ⁻ ⁻, SO₃ ⁻, HSO₄ ⁻,CO₃ ⁻⁻, CO₃ ⁻ ⁻, and HCO₃ ⁻, etc. Moreover, the cathode operates in ahighly oxidizing environment, being polarized to a maximum of about +1 V(vs. the normal hydrogen electrode) while being exposed to pressurizedair. Finally, the anode is constantly exposed to super atmospherichydrogen. Hence, contact elements made from metal must be resistant toacids, oxidation, and hydrogen embrittlement in the fuel cellenvironment. As few metals exist that meet this criteria, contactelements have often been fabricated from large pieces of graphite whichis corrosion-resistant, and electrically conductive in the PEM fuel cellenvironment. However, graphite is quite fragile, and quite porous makingit extremely difficult to make very thin gas impervious platestherefrom.

Lightweight metals such as aluminum and titanium and their alloys havealso been proposed for use in making fuel cell contact elements. Suchmetals are more conductive than graphite, and can be formed into verythin plates. Unfortunately, such light weight metals are susceptible tocorrosion in the hostile PEM fuel cell environment, and contact elementsmade therefrom either dissolve (e.g., in the case of aluminum), or formhighly electronically resistive, passivating oxide films on theirsurface (e.g., in the case of titanium or stainless steel) thatincreases the internal resistance of the fuel cell and reduces itsperformance. To address this problem it has been proposed to coat thelightweight metal contact elements with a layer of metal or metalcompound which is both electrically conductive and corrosion resistantto thereby protect the underlying metal. See for example, Li et al U.S.Pat. No. 5,624,769, which is assigned to the assignee of the presentinvention, and discloses a light metal core, a stainless steelpassivating layer atop the core, and a layer of titanium nitride (TiN)atop the stainless steel layer.

SUMMARY OF THE INVENTION

The present invention comprehends a PEM fuel cell having at least onecell comprising a pair of opposite polarity electrodes, a membraneelectrolyte interjacent the electrodes for conducting ions therebetween,and an electrically conductive contact element confronting at least oneof the electrodes. The contact element has a working face that serves toconduct electrical current from that electrode. The contact elementcomprises a corrosion-susceptible metal substrate, having anelectrically conductive, corrosion-resistant, protective polymer coatingon the working face to protect the substrate from the corrosiveenvironment of the fuel cell. By “corrosion susceptible metal” is meanta metal that is either dissolved by, or oxidized/passivated by, thecell's environment. An oxidizable metal layer may cover a dissolvablemetal substrate, and underlie the conductive polymer layer.

More specifically, the protective coatings of the present inventioncomprises a plurality of electrically conductive, corrosion-proof (i.e.,oxidation-resistant and acid-resistant) filler particles dispersedthroughout a matrix of an acid-resistant, water-insoluble, oxidationresistant polymer that binds the particles together and holds them onthe surface of the metal substrate. The coating contains sufficientfiller particles to produce a resistivity no greater than about 50ohm-cm, and has a thickness between about 5 microns and about 75 micronsdepending on the composition, resistivity and integrity of the coating.Thinner coatings (i.e., about 15-25 microns) are preferred forminimizing the IR drop through the stack. Impervious protective coatingsare used directly on metals that are dissolvable by the system acids.Pervious coatings may be used on metals that are onlyoxidized/passivated, or on dissolvable metals covered with a layer ofoxidizable/passivatable metal.

Preferably, the conductive particles comprise carbon or graphite havinga particle size less than about 50 microns. Most preferably, theparticles comprise a mixture of graphite with smaller carbon blackparticles (i.e., about 0.5-1.5 microns) that fill the intersticesbetween larger graphite particles (i.e., about 5-20 microns) to optimizethe packing density of said particles for improved conductivity. Otheroxidation-resistant and acid-resistant conductive particles may besubstituted for the small carbon black particles. The polymer matrixcomprises any water-insoluble polymer that (1) is resistant to acids andoxidation, (2) can be readily coated or formed into thin films, and (3)can withstand the operating temperatures of the fuel cell (i.e. up toabout 120° C.

The coating may be applied in a variety of ways including: (1)laminating a preformed discrete film of the coating material onto theworking face(s) of the conductive element; or (2) applying (e.g.spraying, brushing, doctor blading etc.) a precursor layer of thecoating material (i.e. a slurry of conductive particles in solvatedpolymer) to the working face followed by drying and curing the film, or(3) electrophoretically depositing the coating onto the working face(s).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will better be understood when considered in the light ofthe following detailed description of certain specific embodimentsthereof which is given hereafter in conjunction with the several figuresin which:

FIG. 1 is a schematic, exploded, isometric, illustration of aliquid-cooled PEM fuel cell stack (only two cells shown);

FIG. 2 is an exploded, isometric view of a bipolar plate useful with PEMfuel cell stacks like that illustrated in FIG. 1;

FIG. 3 is a sectioned view in the direction 3-3 of FIG. 2; and

FIGS. 4 and 5 are magnified portions of the bipolar plate of FIG. 3;

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 depicts a two cell, bipolar PEM fuel cell stack having a pair ofmembrane-electrode-assemblies (MEAs) 4 and 6 separated from each otherby an electrically conductive, liquid-cooled, bipolar plate 8. The MEAs4 and 6, and bipolar plate 8, are stacked together between stainlesssteel clamping plates 10 and 12, and end contact elements 14 and 16. Theend contact elements 14 and 16, as well as both working faces of thebipolar plate 8, contain a plurality of grooves or channels 18, 20, 22,and 24 for distributing fuel and oxidant gases (i.e., H₂ & O₂) to theMEAs 4 and 6. Nonconductive gaskets 26, 28, 30, and 32 provide seals andelectrical insulation between the several components of the fuel cellstack. Gas permeable carbon/graphite diffusion papers 34, 36, 38 and 40press up against the electrode faces of the MEAs 4 and 6. The endcontact elements 14 and 16 press up against the carbon/graphite papers34 and 40 respectively, while the bipolar plate 8 presses up against thecarbon/graphite paper 36 on the anode face of MEA 4, and againstcarbon/graphite paper 38 on the cathode face of MEA 6. Oxygen issupplied to the cathode side of the fuel cell stack from storage tank 46via appropriate supply plumbing 42, while hydrogen is supplied to theanode side of the fuel cell from storage tank 48, via appropriate supplyplumbing 44. Alternatively, air may be supplied to the cathode side fromthe ambient, and hydrogen to the anode from a methanol or gasolinereformer, or the like. Exhaust plumbing (not shown) for both the H₂ andO₂/air sides of the MEAs will also be provided. Additional plumbing 50,52 and 54 is provided for supplying liquid coolant to the bipolar plate8 and end plates 14 and 16. Appropriate plumbing for exhausting coolantfrom the plate 8 and end plates 14 and 16 is also provided, but notshown.

FIG. 2 is an isometric, exploded view of a bipolar plate 56 comprising afirst exterior metal sheet 58, a second exterior metal sheet 60, and aninterior spacer metal sheet 62 inteijacent the first metal sheet 58 andthe second metal sheet 60. The exterior metal sheets 58 and 60 are madeas thin as possible (e.g., about 0.002-0.02 inches thick), may be formedby stamping, by photo etching (i.e., through a photolithographic mask)or any other conventional process for shaping sheet metal. The externalsheet 58 has a first working face 59 on the outside thereof whichconfronts a membrane-electrode-assembly (not shown) and is formed so asto provide a plurality of lands 64 which define therebetween a pluralityof grooves 66 known as a “flow field” through which the fuel cell'sreactant gases (i.e., H₂ or O₂) flow in a tortuous path from one side 68of the bipolar plate to the other side 70 thereof. When the fuel cell isfully assembled, the lands 64 press against the carbon/graphite papers36 or 38 (see FIG. 1) which, in turn, press against the MEAs 4 and 6respectively. For drafting simplicity, FIG. 2 depicts only two arrays oflands and grooves. In reality, the lands and grooves will cover theentire external faces of the metal sheets 58 and 60 that engage thecarbon/graphite papers 36 and 38. The reactant gas is supplied togrooves 66 from a header or manifold groove 72 that lies along one side68 of the fuel cell, and exits the grooves 66 via anotherheader/manifold groove 74 that lies adjacent the opposite side 70 of thefuel cell. As best shown in FIG. 3, the underside of the sheet 58includes a plurality of ridges 76 which define therebetween a pluralityof channels 78 through which coolant passes during the operation of thefuel cell. As shown in FIG. 3, a coolant channel 78 underlies each land64 while a reactant gas groove 66 underlies each ridge 76.Alternatively, the sheet 58 could be flat and the flow field formed in aseparate sheet of material.

Metal sheet 60 is similar to sheet 58. The internal face 61 (i.e.,coolant side) of sheet 60 is shown in FIG. 2. In this regard, there isdepicted a plurality of ridges 80 defining therebetween a plurality ofchannels 82 through which coolant flows from one side 69 of the bipolarplate to the other 71. Like sheet 58 and as best shown in FIG. 3, theexternal side of the sheet 60 has a working face 63 having a pluralityof lands 84 thereon defining a plurality of grooves 86 through which thereactant gases pass. An interior metal spacer sheet 62 is positionedinterjacent the exterior sheets 58 and 60 and includes a plurality ofapertures 88 therein to permit coolant to flow between the channels 82in sheet 60 and the channels 78 in the sheet 58 thereby breaking laminarboundary layers and affording turbulence which enhances heat exchangewith the inside faces 90 and 92 of the exterior sheets 58 and 60respectively.

FIG. 4 is a magnified view of a portion of FIG. 3 and shows the ridges76 on the first sheet 58, and the ridges 80 on the second sheet 60bonded (e.g. by brazement 85) to the spacer sheet 62.

In accordance with the present invention, and as best shown in FIG. 4,the working faces 59 and 63 of the bipolar plate are covered with anelectrically conductive, oxidation resistant oxidation-resistant, andacid-resistant protective coating 94 having a resistivity less thanabout 50 ohm-cm, and comprising a plurality of oxidation-resistant,acid-insoluble, conductive particles (i.e. less than about 50 microns)dispersed throughout an acid-resistant, oxidation-resistant polymermatrix. Preferably, the conductive filler particles are selected fromthe group consisting of gold, platinum, graphite, carbon, nickel,conductive metal borides, nitrides and carbides (e.g. titanium nitride,titanium carbide, titanium diboride), titanium alloyed with chromiumand/or nickel, palladium, niobium, rhodium, rare earth metals, and othernobel metals. Most preferably, the particles will comprise carbon orgraphite (i.e. hexagonally crystallized carbon). The particles comprisevarying weight percentages of the coating depending on the density andconductivity of the particles (i.e., particles having a highconductivity and low density can be used in lower weight percentages).Carbon/graphite containing coatings will typically contain 25 percent byweight carbon/graphite particles. The polymer matrix comprises anywater-insoluble polymer that can be formed into a thin adherent film andthat can withstand the hostile oxidative and acidic environment of thefuel cell. Hence, such polymers, as epoxies, silicones,polyamide-imides, polyether-imides, polyphenols, fluro-elastomers (e.g.,polyvinylidene flouride), polyesters, phenoxy-phenolics,epoxide-phenolics, acrylics, and urethanes, inter alia inter alia areseen to be useful with the present invention. Cross-linked polymers arepreferred for producing impermeable coatings.

The substrate metal forming the contact element comprises acorrosion-susceptible metal such as (1) aluminum which is dissolvable bythe acids formed in the cell, or (2) titanium or stainless steel whichare oxidized/passivated by the formation of oxide layers on theirsurfaces. In accordance with one embodiment of the invention, theconductive polymer coating is applied directly to the substrate metaland allowed to dry/cure thereon. According to another embodiment of theinvention, the substrate metal comprises an acid soluble metal (e.g.,Al) that is covered with an oxidizable metal (e.g., stainless steel)before the electrically conductive polymer topcoat is applied.

The coating may be applied in a variety of ways, e.g., (1)electrophoretic deposition, (2) brushing, spraying or spreading, or (3)laminating. Electrophoretically deposited coatings are particularlyadvantageous because they can be quickly deposited in an automatedprocess with little waste, and can be deposited substantially uniformlyonto substrates having complex and recessed surfaces like those used toform the reactant flow fields on the working face(s) of the contactelements. Electrophoretic deposition is a well-known process useful tocoat a variety of conductive substrates such as automobile and truckbodies. Electrophoretic deposition technology is discussed in a varietyof publications including “Cathodic Electrodeposition”, Journal ofCoatings Technology, Volume 54, No. 688, pages 35-44 (May 1982).Briefly, in electrophoretic deposition processes, a direct current ispassed through a suspension of the conductive particles in an aqueoussolution of a charged acid-soluble polymer. Under the influence of theapplied current, the polymer migrates to, and precipitates upon, aconductive substrate of opposing charge, and carries with it theconductive particles. When cross-linkable polymers are used, thesuspension also includes a catalyst for promoting the cross-linking.Cathodic and anodic electrophoretic processes are both known.Cathodically deposited coatings are preferred for fuel cellapplications, and are deposited by a process wherein positively chargedpolymer is deposited onto a negatively charged substrate. Anodicallydeposited coatings are less desirable since they tend to dissolve someof the substrate metal and contaminate the coating therewith. Incathodic electrophoretic coating, the passage of electrical currentcauses the water to electrolyze forming hydroxyl ions at the cathode andestablishing an alkaline diffusion layer contiguous therewith. Thealkalinity of the diffusion layer is proportional to the cathode currentdensity. Under the influence of the applied voltage, the positivelycharged polymer migrates to the cathode and into the alkaline diffusionlayer where the hydroxyl ions react with the acid-solubilized polymerand cause the polymer to precipitate onto the cathodic substrate. Theconductive filler particles become trapped in the precipitate andco-deposit onto the cathodic substrate. Cathodic epoxies, acrylics,urethanes and polyesters are useful with this method of depositing thecoating as well as other polymers such as those disclosed in the“Cathodic Electrodeposition” publication (supra), and in Reuter et al.U.S. Pat. No. 5,728,283 and the references cited therein. Subsequentbaking of the coated contact element cures and densities the coating.

According to another embodiment of the invention, the coating is firstformed as a discrete film (e.g. by solvent casting, extrusion etc.), andthen laminated onto the working surface of the contact element, e.g., byhot rolling. This technique will preferably be used to make laminatedsheet stock from which the contact elements are subsequently formed,e.g. as by stamping. In this embodiment, the discrete film willpreferably contain a plasticizer to improve handling of the film and toprovide a coating layer atop the substrate that is supple enough so thatit can be readily shaped, (e.g. stamped) without tearing or disruptingthe film when the contact element is formed as by stamping. To insureadherence of the coating to the substrate, the surface of the substrateto which the film is applied is (1) cleaned of all undesirable surfacefilms (e.g., oil), (2) oxides are removed by acid etching, and (3), mostpreferably, roughened or abraded to roughen the surface for anchoringthe film thereto. Fluroelastomers such as polyvinyladiene diflouride orthe like are useful with this embodiment, and may be used withconventional plasticizers such as dibutyl phthalate.

According to another embodiment of the invention, the electricallyconductive polymer film is applied to the working face of the substrateby spraying, brushing or spreading (e.g. with a doctor blade). In thisembodiment, a precursor of the coating is formed by dissolving thepolymer in a suitable solvent, mixing the conductive filler particleswith the dissolved polymer and applying it as a wet slurry atop thesubstrate. The wet coating is then dried (i.e. the solvent removed) andcured as needed (e.g., for thermosets). The conductive particles adhereto the substrate by means of the solvent-free polymer. A preferredpolymer useful with this embodiment comprises a polyamide-imidethermosetting polymer. The polyamide-imide is dissolved in a solventcomprising a mixture of N-methylpyrrolidone, propylene glycol and methylether acetate. To this solution is added about 21% to about 23% byweight of a mixture of graphite and carbon black particles wherein thegraphite particles range in size from about 5 microns to about 20microns and the carbon black particles range in size from about 0.5micron to about 1.5 microns with the smaller carbon black particlesserving to fill the voids between the larger graphite particles andthereby increase the conductivity of the coating compared toall-graphite coatings. The mix is applied to the substrate, dried andcured to provide 15-30 micron thick coatings (preferably about 17microns) having a carbon-graphite content of about 38% by weight. It maybe cured slowly at low temperatures (i.e. <400° F.), or more quickly ina two step process wherein the solvent is first removed by heating forten minutes at about 300° F.-350° F. (i.e., dried) followed by highertemperature heating (500° F.-750° F.) for various times ranging fromabout ½ min to about 15 min (depending on the temperature used) to curethe polymer.

Some coatings may be pervious to the cell's hostile environment.Previous Pervious coatings are used directly only on oxidizable metals(e.g., titanium or stainless steel) and not directly on metals that aresusceptible to dissolution in the fuel cell environment (e.g.,aluminum). Pervious coatings could however be used on dissolvable metalsubstrates (e.g., Al) which have first been coated or clad with anoxidizable/passivating metal layer (e.g., titanium or stainless steel).When pervious coatings are used on an oxidizable/passivating substrateor coating, oxides will form at the sites (i.e., micropores) where thecoating is pervious, but not at sites where the polymer engages thesubstrate metal. As a result, only a small portion of the surface isoxidized/passivated (i.e. i.e., at the micropores in the coating)resulting in very little increase in electrical resistance attributableto the oxide formation.

According to one embodiment of the invention, the electricallyconductive polymer coating is applied to an acid-dissolvable substratemetal (e.g., Al) which had previously been coated with a layer ofoxidizable/passivating metal such as stainless steel. In this regard, abarrier/protective layer 96 of a metal that forms a low resistance,passivating oxide film is deposited onto the substrate 98, and iscovered with a topcoat of conductive polymer 54 in accordance with thepresent invention. Stainless steels rich in chromium (i.e., at least 16%by weight), nickel (i.e., at least 20% by weight), and molybdenum (i.e.,at least 3% by weight) are seen to be excellent such barrier/protectivelayers 96 as they form a dense oxide layer at the sites of themicropores in the polymer coating which inhibits further corrosion, butwhich does not significantly increase the fuel cell's internalresistance. One such stainless steel for this purpose is commerciallyavailable from the Rolled Alloy Company as alloy Al-6XN, and contains23±2% by weight chromium, 21±2% by weight nickel, and 6±2% by weightmolybdenum. The barrier/protective stainless steel layer is preferablydeposited onto the metal substrate using conventional physical vapordeposition (PVD) techniques (e.g., sputtering), or chemical vapordeposition (CVD) techniques known to those skilled in these the art.Alternatively, electrolessly deposited nickel-phosphorous alloys appearto have good potential as a substitute for the stainless steel in thatthey readily form a passivating film when exposed to the fuel cellenvironment which provides a barrier to further oxidation/corrosion ofthe underlying coating.

While the invention has been described in terms of specific embodimentsthereof it is not intended to be limited thereto but rather only to theextent set forth hereafter in the claims which follow.

1. In a PEM fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte intedacent interjacent said electrodes for conducting ions therebetween, and an electrically conductive contact element having a working face confronting at least one of said electrodessfor electrodes for conducting electrical current from said one electrode, the improvement comprising: said contact element comprising a corrosion-susceptible metal substrate and an electrically conductive, corrosion-resistant protective coating on said face to protect said substrate from the corrosive environment of said fuel cell, said protective coating comprising a mixture of electrically conductive particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix and having a resistivity no greater than about 50 ohm-cm, said mixture comprising graphite particles having a first particle size and other electrically conductive particles selected from the group consisting of gold, platinum, nickel, palladium, rhodium, niobium, titanium carbide, titanium nitride, titanium diboride, chromium-alloyed titanium, nickel-alloyed titanium, rare earth metals and carbon, said other particles having a second particle size less than said first particle size to enhance the packing density of said particles.
 2. A fuel cell according to claim 1 wherein said carbon comprises carbon black.
 3. A fuel cell according to claim 1 wherein said coating is electrophoretically deposited onto said substrate from a suspension of said particles in an aqueous solution of acid-solubilized polymer.
 4. A fuel cell according to claim 1 wherein a discrete film of said coating is laminated onto said substrate to form said electrically conductive contact element.
 5. A fuel cell according to claim 1 wherein a precursor of said coating is deposited onto said substrate from a solution thereof, dried and cured to form said coating.
 6. A fuel cell according to claim 1 wherein said substrate comprises a first acid-soluble metal underlying a second acid-insoluble, passivating metal layer susceptible to oxidation in said environment.
 7. A fuel cell according to claim 1 wherein said polymer matrix is selected from the group consisting of epoxies, silicones, polyamide-imides, polyether-imides, polyphenols, fluro-elastomers, polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics and urethanes.
 8. In a PEM fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte intedjacent interjacent said electrodes for conducting ions therebetween, and an electrically conductive contact element having a working face confronting at least one of said electrodes for conducting electrical current from said one electrode, the improvement comprising: said contact element comprising a corrosion-susceptible metal substrate and an electrically conductive, corrosion-resistant protective coating on said face to protect said substrate from the corrosive environment of said fuel cell, said protective coating comprising a plurality of electrically conductive particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix and having a resistivity no greater than about 50 ohm-cm, said substrate comprising a first acid-soluble metal underlying a second acid-insoluble, passivating layer susceptible to oxidation in said environment. 